The present invention relates to deployable structures which include a structural mechanism which is movable for deployment of the structure. A structural mechanism is a mechanism which is mobile without inducing any strain on its components. The deployable structures of the present invention may be used as structures in a wide range of engineering applications ranging from small structures to large ones, for example in aerospace applications. They are particularly, but not exclusively, applicable as frames for tent-like structures.
In general, a structural mechanism may have any number of degrees of mobility, that is degrees of freedom. The present invention relates to the use of a structural mechanism having a single degree of mobility. In general, the advantage of such a single mobility system is that it is much easier to control and therefore more reliable. Having a single degree of mobility avoids the need for complex operations for deployment. Such structural mechanisms having a single degree of mobility, and over-constrained ones in particular, generally have a relatively high stiffness even without the use of latches. These advantages of easy deployment into a particular state, combined with relatively high stiffness in the resultant deployed state, make structural mechanisms particularly useful. Structural mechanisms have particular use to date as precision aerospace structures.
On the other hand, having a single degree of mobility means that any particular structural mechanism can only be deployed in one way. Thus, the configuration on deployment is limited by the design of the structural mechanism itself. Therefore, the utility of structural mechanisms in general is limited by the knowledge of constructions having particular configurations in their undeployed and deployed states.
A number of structural mechanisms are known based on the use of two-dimensional mechanisms as basic elements which are assembled together. To design such structural mechanisms, a suitable two-dimensional mechanism must be identified, together with a construction technique allowing the basic elements to be assembled whilst retaining mobility of each of the basic elements.
An aim of the present invention is to provide a new and useful form of structural mechanism.
According to the present invention, there is provided a deployable structure including a structural mechanism based on Bennett linkages as a basic element.
A Bennett linkage is a three-dimensional, over-constrained linkage consisting of four rigid links connected together in a loop by rotational joints. Each joint connects two links in the loop and provides for rotation of the connected links about an axis of rotation. Bennett linkages are in themselves known. Indeed they were discovered almost a century ago. They are remarkable in that they consist of only four links and yet are mobile by virtue of the joints having axes of rotation in particular directions which are neither parallel nor concurrent.
On movement of a Bennett linkage, the links move in three dimensions. In particular, if one considers the four links as two opposed pair of connected links, movement of the Bennett linkage causes the opposed pairs of links to relatively rotate about an imaginary line through the opposed joints connecting together the two opposed pairs of links. At the same time, those opposed joints move towards or away from each other. Such relative rotation may be visualised by considering the Bennett linkage observed from a position in line with the two opposed joints which connect the two opposed pairs of links.
Bennett linkages have fascinated and challenged kinematicians. However, research to date on Bennett linkage has been concentrated mainly on the design of small linkages based on a Bennett linkage, for example linkages having five or six links.